I am down to installing a subwoofer, does anyone have any pro's or con's in a boat application? I think I can squeeze a 10" in under the dash, and a sealed box will take up less room, but I didnt know if it would be worth the extra room for a ported?

81,
Most contemporary woofers have parameters that are suitable for either sealed (air or acoustic suspension) or ported (bass-reflex) enclosures.
On the average a 10-inch woofer would require a sealed enclosure with 'external' dimensions of 12"x12"x12".
On average a 10-inch woofer would require a bass-reflex enclosure with 'external' dimensions of 16"x16"x16".
The bass-reflex version would deliver 3 dB extra output over the bottom octave of usable bass and have bass extension that reaches 9 Hz lower before the half power point. The extra deep bass extension before roll-off is very audible and the extra 3 dB of output is equivalent to doubling your amplifier power. So bass-reflex has alot of extra acoustic leverage.
Using a well designed bass-reflex alignment that is well damped and linear (versus a one-note resonator for the sole purpose of maximum peak output) will essentially sound equal in sound quality to the right sealed design. But in the context of an open boat environment the best sounding subwoofer is the one where its limits and the amplifier limits are not exposed. This is usually going to be the more responsive bass-reflex system.

OK one more thing, as I understand it, the sound coming from the sub isnt "directional" so if I put it under the dash firing toward the drivers feet basically would this be ok, or should I try and mount it higher up under the dash and have the sub firing at the floor?

81,
Correct, the longer waveforms of the lowest frequencies are non-directional. At the very top of a subwoofer's range and were it overlaps with the in-boat coaxials you will sense some slight directionality in the attack or the transients. What is far more impactful within the realm of a subwoofer is the surrounding planes and loading environment whether it is direct radiating into the cockpit or radiating from within a locker, helm console or other compartment. The results are very predictable but it can get to be a very in depth explanation. Here is a couple of short and simple guidelines:
Reducing the woofer's radiating surface area by TOO tight of a downfiring scenario reduces its output.
Mounting the woofer close to surrounding planes will reinforce the bass.
Placing a woofer in the corner or at one extreme of the cockpit will produce more output.
A woofer/enclosure mounted in a large compartment and venting its radiation through a small surface area opening will reduce the output and change the tonal balance of the woofer providing less in the way of articulation.
Subs when heard alone are rather indiscriminant sounding anyway. Much of the pitch accuracy comes from the midbass produced by the in-boat coaxials. Their installation environment and system tuning is equally important if you want bass with sound quality.

Something on the more practical side to think about with the ported vs. sealed debate is moisture. If you're using a non-marine subwoofer you're probably safer with a sealed enclosure to protect the motor of the woofer.

You didn't mention if this was a DIY build or not. A ported enclosures require more precision to build. Dimensions must be exact and cuts must be precise or you can affect the frequency that it's tuned to. So, if you're planning to whip this out with a drill and circular saw I'd build a sealed enclosure. On the other hand if you're experienced with wood working and have access to a table saw, router, etc then you're equipped to build a proper ported enclosure.